I. Field of the Invention
The present invention is directed toward an ionizing radiation spectroscopy system having a circuit which provides for automatic pole-zero adjustment of that system.
II. Backqround Information
In a typical ionizing radiation spectroscopy system, such as the prior art system 10 illustrated in FIG. 1, incident X-ray, .alpha.-ray or alpha-ray radiation from a radiation source 12 is directed toward an ionizing radiation detector 14. The ionized radiation liberates charge in the form of electron-hole pairs in an electric field within detector 14, which preferably is of a solid state variety.
In addition to detector 14, system 10 includes a preamplifier 16, a high pass filter 18, which includes a capacitor C[1] and a resistor R[1], an amplifier section 20, including an amplifier stage 22 having a feedback resistor and a second stage amplifier/filter 24, and a display 26. The output of detector 14 is coupled to an input of preamplifier 16, whereas the output of preamplifier 16 is connected, through high pass filter 18, to a summing input of amplifier 22, the output of which is connected to the input of amplifier/filter 24. The output of amplifier/filter 24 is connected to the input of display 26. Display 26 comprises a multichannel pulse-height analyzer (MCA) which analyzes and displays the output of amplifier section 20.
In operation of system 10, charge is collected on the detector electrodes of detector 14 to produce an output current pulse with a total charge which is proportional to the energy of the radiation absorbed by detector 14. Preamplifier 16 is charge sensitive and integrates this current pulse to produce a corresponding voltage pulse e[pa](t). Succeeding amplifier 22 and amplifier/filter 24 provide shaping for signal-to-noise enhancement. The peak amplitudes of the processed pulses are digitalized by an analog-to-digital converter within display 26 and a resultant pulse-height distribution is displayed by display 26. This pulse-height distribution may also be stored in memory for later analysis. The value of capacitor C[1] and/or resistor R[]] may be changed to alter the time constant of filter 18.
System 10 of FIG. 1 must add minimum noise to the signal output from detector 14 and must be able to process that signal in the shortest possible time. Preamplifier 16 is optimized for minimum noise. Amplifier 20 provides additional gain and optimized filtering in order to achieve maximum reduction of the preamplifier and detector noise contributions with minimum processing time.
Preamplifier 16 represents a low pass filter with a typical time constant of 50 us to 1 ms. The impulse response of preamplifier 16 is, therefore, a voltage step followed by a decaying exponential, e[pa](t) as shown in FIG. 2. The subsequent high pass filter 18 and amplifier 20 act to differentiate the preamplifier pulse and introduce additional poles at relatively short time constants. The simpliest differentiating network, a high pass filter comprising capacitor C[1] and R[1], has a zero at the origin of the S plane. The use of high pass filter 18 with an exponentially decaying preamplifier signal, such as e[pa](t), produces an undesirable undershoot in the resultant signal E(1)(t), as shown in FIG. 3. Undershoot, as shown in FIG. 3, results in a very long pulse processing times, thus aggrevating pulse-pile up, i.e., the interference by one pulse on the system amplitude determination of successive pulses. In addition, uncompensated singularities, namely the pole of preamplifier 16 and the zero of high pass filter 18, can have a detrimental effect on the noise performance of the overall spectroscopy chain of system 10.
A technique to completely cancel undershoot resulting from high pass filtering of a preamplifier output signal is disclosed by Nowlin et al. in an article entitled "Elimination of Undesirable Undershoot in the Operation and Testing of Nuclear Pulse Amplifiers," Rev. Sci. Instr., Vol. 36, No. 12, December 1965, pp. 1830-1839, the contents of which are incorporated herein by reference. This technique is commonly called pole-zero cancellation. In the differentiation network comprising high pass filter 18, or the electrical equivalent thereof, the zero at the origin of the S-plane is shifted to a location coincident with the pole from the preamplifier. One illustrative network often used for pole-zero cancellation is illustrated in FIG. 4 as comprising resistor R[2] and resistor R[3] coupled to a high pass filter comprising capacitor C[1] and resistor R[1]. As should be apparent to one skilled in the art, by using the network of FIG. 4, an attenuated replica of the output signal from preamplifier 16 may be added to the differentiated signal from the output of high pass filter 18 to exactly cancel the undershoot illustrated in FIG. 3. The resultant output voltage from high pass filter 18, e[1](t) is illustrated in FIG. 5. It should also be apparent to one skilled in the art that the high pass filter 18 of FIG. 1, with capacitor C[1] and resistor R[1] in series, is electrically equivalent to the high pass filter illustrated in FIG. 4 also comprising capacitor C[1] and resistor R[1].
Other configurations have been used in the prior art to accomplish pole-zero cancellation, as is evidenced by the article by Casoli et al. entitled "Active Pole-Zero Cancellation Feedback Loop to Enhance High-Rate Performances of Nuclear Spectroscopy Systems," Nucl. Inst. and Meth., Vol. 156, 1978, pp. 559-566, the contents of which is also incorporated herein by reference.
In order to optimize system performance for a given preamplifier-filter-amplifier combination, the pole-zero adjustment is critical. This adjustment is typically performed by a skilled operator by means of an oscilloscope as evidenced by the above-mentioned Casoli et al. article. The pole-zero adjustment is made difficult due to the follow factors: (1) the noise at the output of the amplifier obscures the effects of any adjustment for undershoot; (2 ) the vertical gain of the oscilloscope must be limited to avoid distortion caused by overload or the input signal must be limited by a resistor-diode clamping network; and (3) the statistical amplitude distribution of the amplifier output pulses masks effects of the pole-zero adjustments. Considerable operator skill is required to make a pole-zero adjustment which results in adequate system performance.
In 1982, in an article by Cova et al. entitled "Automated Regulation of Critical Parameters and Related Design Aspects of Spectroscopy Amplifiers with Time-Invariant Filters," I.E.E.E. Trans. on Nucl. Sci., NS-29(1), Feb. 1982, pp. 609-613, a circuit was introduced for assisting in manually adjusting a pole-zero cancellation circuit. A similar disclosure appears in U.S. Pat. No. 4,491,799 issued to Giardinelli. The content of both these documents is incorporated herein by reference.
The circuit disclosed by Cova et al. and Giardinelli eliminates the need for an oscilloscope, eliminates the need for a limiting network and reduces the skill required in pole-zero adjustment of the system. The circuit comprises a boxcar averager, two light-emitting diodes (LEDs), and associated control circuitry. The boxcar averager is used to sample the base line at the output of the amplifier, such as the output of amplifier 20 of FIG. 1, after each pulse which is validly detected by detector 14. The LEDs are then used to indicate whether the resultant pulse, e[1](t), possesses an undershoot as shown in FIG. 3 or an overshoot, with one of the LEDs being lit in the presence of an undershoot, and the other of the LEDs being lit in the presence of an overshoot. Adjustment is then made in a pole-zero compensation circuit of the type illustrated in FIG. 4, until both LEDs are turned off. However, with this technique the setting of pole-zero compensation is still a manual adjustment, requiring the operator to turn a potentiameter until proper compensation is achieved.
An object of the present invention is to provide an automatic pole-zero adjustment circuit for an ionizing radiation spectroscopy system.
Additional objects and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention.